Interleukin 15 (IL-15) is a molecule that is highly relevant to natural killer (NK) cell development and homeostasis. This proposal is now in its third cycle of competitive funding and focuses on IL-15 and its role in human NK cell development, effector function and cancer. Our specific aims remain relatively unchanged: 1) To further identify and characterize progenitors and precursors that differentiate into human NK cells; 2) To better understand the role of IL-15 in regulating human NK cell effector functions; 3) To identify and assess the mechanism(s) by which deregulation of endogenous IL-15 leads to the induction of acute T and NK cell large granular lymphocyte (LGL) leukemia, and to develop a therapeutic strategy that effectively cures this leukemia. The immediate past cycle of funding has been marked by a series of novel findings that include (but are not limited to) the in situ discovery of a CD34(+) human NK cell progenitor population as well as the successive stages of human NK cell intermediaries within secondary lymphoid tissue; a novel immature NK cell in mucosal associated lymphoid tissue that constitutively expresses IL-22, a cytokine that is important for epithelial cell secretion of anti-microbial peptides; a novel role for SET, an inhibitor of the phosphatase PP2A, in modulating NK cell interferon gamma (IFN-?) secretion; and a process called reciprocal antagonism, whereby NK cell pro-inflammatory signaling simultaneously suppresses its own constitutive anti-inflammatory signaling, and vice-versa. Finally, we discovered that IL-15-mediated leukemogenesis involves a process of non-random methylation across the genome, a process that silences putative tumor suppressor genes and is highly responsive to hypomethylation therapy. In the current proposal, we provide preliminary data supporting a role for dendritic cell IL-1? as critical to the homeostasis of the IL-22-secreting immature NK cell and explore its differentiation to IFN-?-secreting NK cell; we explore the role of at least two factors, RUNX2 and SET, in mediating IL-15 NK cell effector function, and we provide preliminary data suggesting that excessive methylation in IL-15-mediated leukemogenesis may be related to alteration of microRNA that regulate DNA methyltransferases. The latter discovery not only promises to provide insight into mechanism of disease but also therapeutic strategies to build upon our early success with hypomethylation therapy for LGL leukemia. While this proposal continues to contain basic and pre-clinical experimentation, the principal investigator has used discoveries from this and other work to enter hundreds of cancer patients onto clinical trials. The National Cancer Institute will soon make IL-15 available to experienced clinical investigators for early phase clinical trials in cancer patients, and work outlined in this proposal will likely bear fruit for such trials. It will also likely be useful for modulating the immune system in treating acute myeloid leukemia, for providing new insights into mucosal immunity that will likely be important for oral vaccine development, and for establishing new preclinical data directing us toward successful treatment of acute LGL leukemia, currently a uniformly fatal disease.